The field of the invention is devices for optimizing the discharge of electrochemical cells. More particularly, the present invention concerns an electrochemical cell discharge device applicable when heavy currents are required to supply apparatus.
As is known, electrochemical cells are formed by two electrodes disposed in an electrolyte and they may be batteries (primary cells) or accumulators (secondary cells). They are characterized by a nominal voltage and a capacity expressed in Ah.
In conventional manner the discharge of an electrochemical cell is effected with continuous current. The capacity of a cell depends in particular on the discharge rate to which it is subjected. The capacity of a cell falls when its discharge rate increases (Peukert's law). This decrease is due to the diffusion of ions in the electrolyte and/or in the solid phases of the electrode materials under steady loading.
One consequence of a high discharge rate is the establishment of concentration gradients of electro-active species within the electrodes. This is the result of heterogeneous behavior (local over-discharge or overload) and poor use of the active materials, resulting in a lowering of the instantaneous capacity and a reduction in the working life of the electrochemical cell.
It is thus preferable to discharge an electrochemical cell in a pulsed mode in which the relaxation induced in each rest period allows the behavior of the electrodes to be homogenized.
Discharge in a pulsed mode is used especially for heavy current applications. It is thus possible to feed apparatus with the aid of switching supplies or choppers. However, the main problem with this type of supply is that the frequency at which the continuous supply voltage is chopped is too high (from some kHz to some hundreds of kHz) to allow such homogenization. It is not possible to reduce the chopping frequency too much, because of the inevitable increase in the size of the capacitors serving to smooth the chopped voltage. These capacitors usually act in conjunction with a diode and an inductance (.pi. network).
Increase in the size of the capacitors involves a higher cost price for the supply and can be contemplated only within certain limits and for site applications. It is thus not possible to use large capacitors in transportable apparatus of the kind for portable use (cordless drills, etc.), electric motor vehicles, radio-telephones or first aid equipment (defibrillators, etc.).
In known manner, the optimum periods of pulsed discharge are high and in the order of some seconds to some tens of seconds and at such frequencies capacitors do not exist enabling a continuous current to be obtained which is directly usable for on-board applications.
A discharge device for electrochemical cells is described in German patent application 26 05 730 in the name of Licentia Patent Verwaltungs GmbH. This device, applied for example to the discharge of two electrochemical cells in an apparatus fed by these cells, makes alternate connection of each of the electrochemical cells to the supply terminals of the apparatus. Each cell cooperates with a first terminal of switching means forming a circuit breaker, the second terminal of each switching means being connected to the supplied apparatus. The second terminals of the switching means are connected together. The switching means are controlled by digital control signals output by a pulse train generator. Each switching means is rendered conducting when a pulse is applied thereto, which causes the associated electrochemical cell to be connected to the supplied apparatus. The pulses of the two trains are interleaved (i.e., do not overlap) in time, so that the two cells will never be connected simultaneously to the apparatus. One of the trains is obtained by logical inversion of the other.
However, the presence of a maintaining capacitor connected in parallel with the supplied apparatus is essential, since the pulses of the two trains are not strictly complementary, because of the logical inversion and the delay introduced by the logical inversion. The result is that, at each switchover, there is a period of time during which neither of the cells is connected to the supplied apparatus, the maintaining capacitor taking over the function of a cell during these intervals of time. However, if the apparatus requires a heavy supply current, this capacitor likewise has to have a large capacitance. The devices described in this document are thus not suitable for on-board applications with a heavy discharge current.